The present invention relates to multi-point visual communication systems which connect a plurality of sites for two-way communication of information, more particularly, to a multi-point visual communication system which transmits a large quantity of data such as video or voice information through a predetermined transmission line. The present invention also relates to a multi-point visual communication system which allows control of video and voice quality at signal transmitting and/or receiving side and to a visual signal processing system.
Multi-point visual communication is carried out under such a condition that visual communication systems installed at a plurality of sites are connected to each other via a telecommunication network. There are various types of multi-point visual communication. The visual communication system has a function of subjecting a video signal issued from such a video equipment as a television camera to an analog modulation or digital coding to obtain a modulated or coded signal and then sending the modulated or coded signal to the telecommunication network. The visual communication system also has a function of receiving from the telecommunication network video information sent from the other visual communication systems installed at different sites, reproducing the received video information into a video signal and then outputting a reproduced video signal to such a video equipment as a display unit. The telecommunication network connecting these visual communication systems enables the video information sent from the visual communication systems installed at the respective sites to be distributed to the visual communication systems installed at other sites.
In the prior art multi-point visual communication, the quantity of video information is increased as the number of sites to be connected through the telecommunication network increases, which results in that the communication systems must be high in performance thereby becoming expensive. As the charge for use of the telecommunication network is increased with increased use frequency of the telecommunication network, use of such systems by general users to some extent decreases.
For the purpose of solving these problems, there have been proposed various devices for reducing the quantity of video information in the prior art.
Disclosed, for example, in JP-A-63-276938 is an example of multi-point visual communication in which the number of picture elements (pixels) in a video signal is reduced to one divided by the number of the parties and then the pixel-reduced video signal is subjected to a coding operation. FIG. 10 is a diagram for explaining the above prior art. More specifically, FIG. 10 shows an example in which visual communication is carried out under such a condition that 5 sites are connected for video display. In this connection, the number of the parties with respect to a terminal in this site is 4. Visual communication terminals installed at the respective sites transmit video information signals 94 to 98 which correspond to reduction of video signals in the pixel number to one divided by the number of the parties (that is, 1/4 in this example). The video information signals transmitted from the parties are combined at the terminal in this site as a signal receiving point and displayed on a display screen 99 of m.times.n pixels of the terminal in this site.
JP-A-63-121374 also discloses a digital coding technique for controlling the quantity of video information according to characteristics such as video motion or inter-frame variation. More in detail, in this case, a low pass filter is provided for cutting off high spatial resolution components so that the ON and OFF operation of the low pass filter is controlled according to the video motion.
Even in a paper reported in Proceedings D-228 of the Autumn General Meeting of The Institute of Electronics, Information and Communicate Engineers of Japan, there is reported an adaptive coding control system for controlling the high-cut performance of a low pass filter according to a variation in the inter-frame of a video signal.
The aforementioned prior art systems have problems which follow. That is, such a prior art multi-point visual communication as mentioned above has a problem that, since increase of the number of sites to be connected causes decrease of the number of pixels in a video signal, the quality of the video signal transmitted from each site is deteriorated. In addition, since the number of pixels in the video signal to be transmitted is uniformly determined, the video quality satisfying users' demand cannot be always obtained.
Further, such video information quantity control as in the prior art has a problem that since the characteristics (variation in the inter-frame of a video signal) alone of a video signal are estimated to suppress a spatial resolution, the above control is unsuitable for dynamic video signal transmission with the truly necessary spatial resolution maintained, which results in that the information quantity control cannot be carried out according also to the situations of a telecommunication network.